This invention pertains to the field of ionizing gas samples for analysis, and in particular to reducing the residual current in an ionization gauge or mass spectrometer ion source due to the X-ray effect by using a louvered electron beam stop.
The low pressure measurement limit of total pressure gauges is largely limited by the generation of x-rays when electrons strike grid surfaces in the gauge structure. The x-rays generated have an angular distribution of intensities leaving the surface with a maximum intensity near the normal. Some of these energetic photons can strike the total pressure collector and eject an electron from the ion collector. This loss of an electron is indistinguishable from the arrival of a positive ion at the collector. Thus, the photoelectron ejection is recorded as an ion current which at very low pressures produces an xe2x80x9cX-ray limitxe2x80x9d for low pressure for any gauge. This X-ray limit is particularly high for the triode type of ion gauge where the collector surface is a cylinder around the central filament and grid. Prior art in minimizing this problem includes:
1. Making the total pressure collector a very fine wire to minimize the area for x-ray absorption (the Bayard-Alpert Ion Gauge - (BAG),
2. Modulating the ion collection to reject photoelectron effects (Modulated BAG),
3. Extracting the ions formed in a gauge through an aperture to a separate collection region using a modulated ion repeller (The Extractor Gauge of Redhead, et al.), and
4. Extracting the ions and bending them with an electrostatic analyzer to an ion collector (The Helmer Gauge).
All of these gauges focus on minimizing the incidence of the x-rays present in a standard cylindrical grid structure by designing a particular size and location of the collector element or modulating the ion collection process.
Briefly stated, in an ionization gauge, the effect of X-rays emitted when a collimated electron beam strikes grid surfaces in the gauge structure is reduced by a louvered beam stop. The louvered beam stop creates shadow regions having no X-rays, thus minimizing the amount of X-rays striking the collector plate and reducing the X-ray effect portion of the residual current.
According to an embodiment of the invention, in an ionization gauge having an electron beam shaped by an anode, in which gas molecules are ionized by the electron beam for analysis, and a beam stop which collects substantially all electrons in the electron beam not interacting with the gas molecules, the beam stop includes at least one louver on the beam stop.
According to an embodiment of the invention, an apparatus for determining a total ion pressure of a gas includes an ionization chamber; the ionization chamber having first and second ionization regions, wherein a boundary between the regions is defined by an anode grid or aperture; means for producing an electron beam passing through the first and second ionization regions, whereby an interaction between the electron beam and molecules of the gas within the ionization chamber produce first and second ion streams from a same gas density; an electron beam stop which collects substantially all electrons not interacting with the gas molecules, the electron beam stop including at least one louver; means for directing the first ion stream to an analyzer; and means for directing the second ion stream to an ion collector.
According to an embodiment of the invention, a method for determining a total ion pressure of a gas includes (a) providing an ionization chamber, the ionization chamber having first and second ionization regions, wherein a boundary between the regions is defined by an anode grid or aperture; (b) producing an electron beam passing through the first and second ionization regions, whereby an interaction between the electron beam and molecules of the gas within the ionization chamber produce first and second ion streams from a same gas density; (c) using an electron beam stop which collects substantially all electrons not interacting with the gas molecules, the electron beam stop including at least one louver; (d) directing the first ion stream to an analyzer; and (e) directing the second ion stream to an ion collector.
According to an embodiment of the invention, a method for reducing a residual current in an ionization gauge includes (a) providing an ionization chamber containing gas molecules of a gas to be analyzed; (b) producing an electron beam passing through the ionization chamber, whereby an interaction between the electron beam and the gas molecules within the ionization chamber produce at least one ion stream; (c) directing at least a portion of the ion stream to an ion collector; and (d) using an electron beam stop which collects substantially all electrons not interacting with the gas molecules, the electron beam stop including at least one louver, wherein the at least one louver directs a plurality of X-rays formed by the electrons interacting with the electron beam stop away from the ion collector.